The invention relates to a particle size analyzer based on a laser diffraction method, more particularly, a particle size analyzer capable of accurately measuring a wide range of particles having a fine diameter of the order of 0.1 xcexcm to a large diameter of the order of several thousands xcexcm.
In a particle size analyzer based on the laser diffraction method, generally, a particle size distribution of particles to be measured can be calculated through measuring a space intensity distribution of diffracted/scattered light obtained by irradiating laser light to the particles to be measured in a dispersing/flying state; and calculating the measured results based on a Mie""s scattering theory and a Fraunhofer""s diffraction theory.
More specifically, in a basic structure of a measuring portion of the measuring apparatus of this type as diagrammatically shown in FIG. 6, when laser beam from a laser light source 41 is irradiated to a particle group P to be measured through a collimating lens 42 to be parallel beam, the laser beam is diffracted or scattered by the particle group P to be measured to thereby form a spacial light intensity distribution pattern. Among the diffracted/scattered light (hereinafter simply referred to as xe2x80x9cscattered lightxe2x80x9d), the forward scattered light is converged by a lens 53a to form ring-shape scattered images on a detection plane disposed at a focal distance position. The forward scattered light intensity distribution pattern is detected by a ring detector (forward scattered light sensor) 53b formed of a plurality of light sensor elements having ring-shape light receiving surfaces of different radii arranged concentrically. Also, the sideward and backward scattered light is detected by sideward scattered light sensors 54 and backward scattered light sensors 55.
The space intensity distribution pattern of the scattered light measured at the measuring portion by the plural light sensors is digitized by the A/D converter and inputted to a computer as the scattered light intensity distribution data.
The scattered light intensity distribution data are varied depending on the size of the particles. Since different size particles are mixed in the actual particle group P to be measured, the intensity distribution data of the scattered light by the particle group P are formed by laying one scattered light on top of the other, respectively. When this situation is expressed by a matrix, Equation (1) can be obtained:
s=Aqxe2x80x83xe2x80x83(1)
wherein,                               s          =                      (                                                                                s                    1                                                                                                                    s                    2                                                                                                ⋮                                                                                                  s                    m                                                                        )                          ,                  xe2x80x83                ⁢                  q          =                      (                                                                                q                    1                                                                                                                    q                    2                                                                                                ⋮                                                                                                  q                    n                                                                        )                                              (        2        )                                A        =                  (                                                                      a                                      1                    ,                    1                                                                                                a                                      1                    ,                    2                                                                              ⋯                                            ⋯                                                              a                                      1                    ,                    n                                                                                                                        a                                      2                    ,                    1                                                                                                xe2x80x83                                                                              xe2x80x83                                                                              xe2x80x83                                                                              xe2x80x83                                                                                    ⋮                                                              xe2x80x83                                                                              a                                      i                    ,                    j                                                                                                xe2x80x83                                                                              xe2x80x83                                                                                    ⋮                                                              xe2x80x83                                                                              xe2x80x83                                                                              xe2x80x83                                                                              xe2x80x83                                                                                                      a                                      m                    ,                    1                                                                              ⋯                                            ⋯                                            ⋯                                                              a                                      m                    ,                    n                                                                                )                                    (        3        )            
s (vector) is intensity distribution data (vector) of the scattered light. Their elements si (i=1, 2, . . . m) are incident light quantities detected by the respective elements of the ring detector 53b, and sideward and backward scattered light sensors 54, 55.
q (vector) is particle size distribution data (vector) expressed as a frequency distribution %. A region of diameters of the particle group to be measured (maximum particle diameter: X1, minimum particle diameter: Xn+1) are divided into n, and the respective particle diameter intervals are represented by (Xj, Xj+1) (j=1, 2, . . . n). The elements qj (j=1, 2, . . . n) of the q (vector) are the particle quantities corresponding to the particle diameter intervals (Xj, Xj+1). Normally, it is normalized to be the following Equation (4).                                           ∑                          j              =              1                        n                    ⁢                      q            j                          =                  100          ⁢          %                                    (        4        )            
A (matrix) is a coefficient matrix for converting the particle size distribution data (vector) q to light intensity distribution data (vector) s. The physical meaning of elements ai,j (i=1, 2, . . . m, j=1, 2, . . . n) of A (matrix) is an incident light quantity with respect to the i-th element of light scattered by the particles of a unit particle quantity belonging to the particle diameter interval (Xj, Xj+1).
A numeral value of ai,j can be theoretically calculated beforehand. In case the diameter of the particle is sufficiently large when compared with a wavelength of the laser beam as the light source, Fraunhofer""s diffraction theory is used. However, in a region where the diameter of the particle is shorter than or the same as the wavelength of the laser beam, i.e. sub-micron region, it is necessary to use Mie""s scattering theory. The Fraunhofer""s diffraction theory can be considered to be an excellent approximate of the Mie""s scatting theory effective in case the particle diameter is sufficiently large when compared with the wavelength in a forward fine-angle scattering.
In order to calculate the elements of a constant matrix A by using the Mie""s scatting theory, it is necessary to set the absolute refractive indexes, i.e. complex numbers, of particles and a medium, i.e. medium liquid, in which the particles are dispersed. There may be a case where a relative refractive index, i.e. complex number, of the particles and the medium is set, instead of setting the respective refractive indexes.
As an equation for obtaining the least square integral of the particle size distribution data (vector) q based on the above equation (1), the following equation is obtained:
xe2x80x83q=(ATA)xe2x88x921ATSxe2x80x83xe2x80x83(5)
wherein, AT is a transpose of A, and ( )xe2x88x921 is an inverse matrix.
The respective elements of the light intensity distribution data (vector) s in the right side of Equation (5) are numeral values detected by the ring detector 53b, sideward scattered light sensors 54 and backward scattered light sensors 55. Also, the coefficient matrix A can be obtained beforehand by using the Fraunhofer""s diffraction theory or the Mie""s scattering theory. Therefore, when the calculation of Equation (5) is carried out by using the known data, it is apparent that the particle size distribution data (vector) q can be obtained.
The above explanation is a basic measuring theory of the particle size distribution measurement based on the laser diffraction method. Incidentally, the above-explained method is one example of methods for calculating the particle size distribution, and there are many other variations in measuring methods and kinds and arrangements of the sensors and detectors.
Here, in the conventional particle size analyzer based on the laser diffraction method, a laser beam source having a wavelength of 600 to 800 nm is used as a laser beam source 41, as shown in FIG. 6. With the laser beam having such a wavelength, a particle size distribution in a particle diameter region of the order of sub-sub-micron less than 0.1 xcexcm can not be measured.
A specific structure of a conventional particle size analyzer based on the laser diffraction method is shown as a block diagram in FIG. 7. The particle size analyzer in this example comprises an irradiation optical system 51 including a semiconductor laser 51a and a collimating lens 51b to allow output beam therefrom to become parallel beam; and a measurement optical system 56 including a flow cell 52 wherein a suspension in which a sample P to be measured is dispersed in a medium liquid flows, a forward scattered light sensor formed of a converging lens 53a for detecting only the light diffracted/scattered in a predetermined front angle region among the diffracted/scattered light by the particle group P to be measured and a ring detector 53b, sideward scattered light sensors 54 and backward scattered light sensors 55 for detecting the light diffracted/scattered sideward and backward among the light diffracted/scattered by the particle group P to be measured, respectively. Then, outputs from the respective light sensors are amplified and digitized at a data sampling circuit 57 including amplifiers and A/D converters corresponding thereto, and then inputted to a computer 58. In the computer 58, a particle size distribution of the particle group P to be measured can be obtained through the above-stated operation by using a space intensity distribution data of the diffracted/scattered light formed of the entire outputs of the respective light sensors, and the results are outputted to a display device 59 or a printer 60 to display or print out thereof.
Here, as a light source of the irradiation optical system 51, a laser other than the semiconductor laser 51a as in the above example may be used. However, in either case, a wavelength of the output beam is in a range from 600 to 800 nm.
Incidentally, in the conventional particle size analyzer as shown in FIG. 7, the reason why the sideward and backward scattered light sensors 54 and 55 are provided in addition to the forward scattered light sensor 53 as the measurement optical system 56 is to make the measuring lower limit of the particle size distribution smaller, so that the measuring region is extended to a particle size of the order of sub-micron. In other words, there is a tendency such that in the space intensity distribution of the light scattered by the particle group to be measured, as the particle diameter becomes smaller, the beam intensity of a large scattering angle becomes stronger. Thus, in case particles with small diameters are measured, it is necessary to measure the space intensity distribution of the scattered light in a wide angle range by also detecting the sideward and backward scattered light. In case a laser beam having a wavelength of the order of 600 to 800 nm is used as a light source of the irradiation optical system as in the conventional apparatus, without the backward and sideward scattered light sensors, particles having diameters of the order of sub-micron can not be measured.
Thus, in the conventional particle size analyzer wherein particles having diameters of the order of sub-micron can be measured, as shown in FIG. 7, in addition to the forward scattered light sensor, it is essential to provide the sideward and backward scattered light sensors, so that the structure of the measurement optical system becomes complicated and the cost of the apparatus is raised.
Also, as the forward scattered light sensor, in case the converging lens and the ring detector are used, only if an optical axis coincides with respect to the irradiation optical system, an accurate measurement can be made. However, in the sideward and backward scattered light sensors, since it is necessary that each light sensor is accurately set in an angle with respect to the light irradiation position against the particle group to be measured, it is required to hold a fixed relative positional relationship of the sample cell containing or flowing therethrough the particle group to be measured with the irradiation optical system and the measurement optical system. For example, in case particles with the diameters in the order of sub-micron floating in air or water are measured, there has been a problem wherein a measurement in an open system without using the sample cell can not substantially be carried out.
Also, as shown in FIG. 8, a particle size analyzer has been practically used, which includes two systems, i.e. an irradiation optical system and a measurement optical system, enabling to measure particles having diameters less than 0.1 xcexcm.
More specifically, as the irradiation optical system, in addition to a semiconductor laser 51a having a wavelength region as described above, there are provided a light source 61 capable of outputting beam having a wavelength shorter than the laser and a filter 61a, so that the light sources 51a and 61 selectively irradiate a sample cell 52 wherein a suspension in which a particle group P to be measured is dispersed in a medium flows or is contained. At the same time, in addition to a forward scattered light sensor 53b, sideward scattered light sensors 54 and backward scattered light sensors 55 which are the measurement optical system for the semiconductor laser light source 51a, there are provided sideward scattered light sensors 62 and backward scattered light sensors 63 which are the measurement optical system for the light source 61, and outputs from the respective light sensors are inputted to a computer 58 through a data sampling circuit 57 including amplifiers and A/D converters corresponding to the respective sensors. In the computer 58, a particle size distribution in a region of particle diameters of the order of sub-micron is calculated from space intensity distribution data of the scattered light obtained from the semiconductor laser 51a and the measurement optical system corresponding thereto, and at the same time a particle size distribution having smaller particle diameters in the order of sub-sub-micron is calculated from the space intensity distribution data of the scattered light obtained from the light source 61 and the measurement optical system corresponding thereto.
Also, as an apparatus based on another method for measuring particle diameters in a region less than 0.1 xcexcm, an apparatus has been practically used, wherein a particle size analyzer based on the light diffraction method using a laser beam source having a red color wavelength region is used in combination with a photon correlation method, i.e. dynamic light scattering method, taking advantage of a periodic information of a Brownian movement.
All the conventional particle size analyzer enabling to measure particle diameters in a region of the order of sub-sub-micron can not accurately measure a particle size distribution in a particle group to be measured. More specifically, in the apparatus including the above-described two systems, such as the irradiation optical system and the measurement optical system, or the particle size analyzer wherein the measuring method based on a theory different from the laser diffraction method is jointly used, the measured results of the particle size distribution have break-points without fail, so that accurate particle size distribution of the particle group to be measured can not be obtained. Here, since the particle size distribution generally relies on its measuring method, even if measured results obtained based on different measuring methods are combined together, the result is meaningless.
Also, in case particles of a pigment, dye or ink are measured, the blue color particles easily absorb the red color laser beam and, especially, particles in a region of sub-micron have such a conspicuous tendency. Therefore, there has been a problem such that an accurate particle size distribution of a blue color particle group can not be measured by the conventional particle size analyzer using irradiation of the red color laser beam.
An object of the present invention is to provide a particle size distribution measuring apparatus, wherein even if a structure of a measurement optical system is simplified, particles having diameters of the order of sub-micron can be measured as in the conventional measuring apparatus, so that although the particle size analyzer of the invention has the same performance as that of the conventional measuring apparatus, the apparatus cost can be reduced.
Another object of the invention is to provide a particle size analyzer, wherein even if the measurement optical system and the irradiation optical system are separated, particles having diameters in the order of sub-microns can be measured, so that the particles having diameters of the order of sub-micron can be measured in an open system without using a cell.
A further object of the invention is to provide a particle size analyzer based on the laser diffraction method, wherein a particle size distribution of a particle group to be measured including even blue color particles can be accurately measured continuously in a wide range of the order from a sub-sub-micron to several thousands of xcexcm.
In order to attain the above objects, a particle size analyzer based on the laser diffraction method of the invention includes an irradiation optical system for irradiating laser beam to a particle group in a dispersing state; a measurement optical system for receiving light, diffracted/scattered by the particles, of the laser beam irradiated from the irradiation optical system, and measuring a space intensity distribution; and an operating portion for calculating a particle size distribution of the particle group from the measurement results obtained at the measurement optical system, wherein a light source in the irradiation optical system is a semiconductor laser having an output beam wavelength in a range of from 300 to 500 nm.
In the invention, only a laser diffraction method is used; laser beam of a single wavelength in a range of 300 to 500 nm is used as light to be irradiated to the particle group to be measured; and the space intensity distribution of the light diffracted/scattered, by the particle group to be measured, of the laser beam is measured to thereby attain a desired object.
More specifically, in case a measuring angle range of the diffracted/scattered light from the particle group to be measured is fixed, the minimum value of the particle diameter to be measured can be made smaller as a wavelength of the beam to be irradiated to the particle group to be measured is shorter. When the laser beam having a wavelength in a region of 300 to 500 nm is used as the irradiation beam to be irradiated to the particle group to be measured, by measuring only the space intensity distribution of the diffracted/scattered light in the same measuring angle region as in the conventional technique, the particle size distribution in a wide range from a sub-sub-micron to several thousands xcexcm can be accurately measured without break-points. Also, the laser beam having a wavelength from 300 to 500 nm is less absorbed by the blue color particles. Thus, with respect to the blue color particles, by measuring the space intensity distribution of the diffracted/scattered light in the same measuring angle region as in the conventional technique, an accurate particle size distribution can be obtained in the above-stated wide particle diameter region.
In the invention, in addition to the basic structure as explained above, the measurement optical system may be formed of a plurality of light sensors disposed in an area of a predetermined front angle in an irradiating direction of the laser beam.
Also, the measurement optical system may be formed of a converging lens for converging light diffracted/scattered by the particle group to be measured, and a ring detector disposed at a focal position of the converging lens and formed of a plurality of independent conical light receiving surfaces in the shape of a ring, half ring or xc2xc ring having different radii.
Further, in case the converging lens and the ring detector are used as the forward scattered light sensors for forming the measurement optical system, only the optical axes of the converging lens and the ring detector are aligned with the optical axis of the irradiation optical system. As a result, it is possible to precisely measure the space intensity distribution of the diffracted/scattered light. Since the mutual positional relationship between the irradiation optical system and the measurement optical system need not be held constantly through the sample cell as in the case where the sideward and backward scattered light sensors are used. Thus, it is possible to measure particle size distribution in air or water in the order of sub-micron with an open system.